Metallic screws have been used for tissue fixation in surgical procedures since he 1940's. Metallic screws preferably made of stainless steel or titanium are still most commonly used in surgery. Metallic materials, however, are significantly stiffer than bone or other tissues in human body. This mechanical incompatibility of the fixation device with the healing tissue generates several problems, such as a stress shielding effect, the loosening of the fixation due to micro motion on the implant tissue interface, and the local necrosis of the tissue due to too high local stresses in the tissue. Additionally, long term complications due to the corrosion products of the fixation devices are not uncommon.
Bioabsorbable fixation devices theoretically overcome all the above mentioned problems related to metallic fixation devices. The modulus of the polymer materials can be tailored to be nearly equivalent with the tissues related to a specific application, which leads to accelerated healing due to biomechanically optimised stress transfer to the healing tissue. Bioabsorption, on the other hand, diminishes the risk of long term complications.
Bioabsorbable fixation devices have not replaced the metallic fixation devices used in surgery as was expected already years ago. There are several reasons for that, such as the lack of confidence on the bioabsorbable products among the clinicians used to perform the operations with metallic devices, and in some cases, the more demanding surgical technique related to the use of the bioabsorbable fixation device. In the case of bioabsorbable fixation screws, the screw head design is strongly limited by the mechanical properties of the bioabsorbable polymer materials. The material properties of bioabsorbable polymers do not allow the usage of standardised bone fixation screw heads, e.g. hexagonal or torx heads.
Most surgical screws have been made of metallic materials, e.g. titanium or stainless steel. These screws have multiple thread and head configurations to be used with different kinds of instrumentation. To ease the work of the orthopedic surgeons, AO/ASIF (Arbeitsgemeinschaft für Osteosynthesefragen/Association for the Study of Internal Fixation) has published proposed configurations of general surgical screws to unify the instruments used in surgery. The surgeons could then use the same instrumentation, drills, taps etc. with implants of different manufacturers, if the implants have been made according to the AO/ASIF proposal. The proposal has also been turned to the ISO standards ISO 5835, ISO 6475 and ISO 9268 and to the ASTM standards ASTM F116-00 and ASTM F543-02.
The AO/ASIF proposal comprises multiple different shapes for the slot of the screw head, such as cruciform, square and hexagonal. The most used one is the hexagonal head. There are also holding sleeves to be attached to the screwdriver, which holds the screw in the driver during the implantation. The AO/ASIF screwdrivers with the holding sleeves have been designed to be used with standard screws.
There are also bioabsorbable screws in use, which are absorbed in the human body after the implantation so that a removal operation is not needed. The problem with AO/ASIF compatible instruments and bioabsorbable screws is that the instruments have been designed to be used with metallic implants. The mechanical properties of the bioabsorbable screws are not adequate to be able to use the AO/ASIF instruments with them. Usually, the bioabsorbable screws have to be implanted with appropriate instrumentation that increases the costs of the operations. Adapters which have been attached to the screw but which only fit the appropriate instruments, have been invented in several cases, see for example U.S. Pat. No. 5,971,987 and EP 0276153. Also, an adapter that fits between the screw and the AO/ASIF instrument has been invented, see U.S. Pat. No. 5,868,749, but the invention has not included the holding sleeve that should also fit the adapter, to be used with it. Moreover, the adapter in question is intended to be used only with screws made of bone substance.
The other problem with the use of bioabsorbable screws is the breaking off of the screw when twisted with the screwdriver. Because surgeons are used to applying metallic screws that can withstand much more torque, the shift to bioabsorbable screws often results in using excessive torque with them. There are torque-limiters invented to be used with both metallic and bioabsorbable screws. Some of them are mechanisms built in the screwdrivers so that the driver gives in when twisted with too much torque (see U.S. Pat. No. 6,132,435). The forces of such torque-limiters have been adjusted to metallic screws and cannot be used with bioabsorbable ones. In many cases, they have also been designed to be used with electrical screwdrivers, which are not appropriate for bioabsorbable screws because it is important to use manual drivers to maintain a better response when tightening bioabsorbable screws. There are also torque-limiters for metallic and bioabsorbable screws that are integrated in the screw (see EP 0276153 and U.S. Pat. No. 6,471,707). They function so that there is a weaker score in the screw head for the purpose that the head will break off when excessive torque is applied. The screw has to be tightened with a secondary instrument, if the screw has not been tightened enough until the head breaks off. Also, if the screw has been stuck, the secondary instrument has to be used to remove the screw.